IEEE Power Electronics Magazine - December 2021 - 25

TT T,,ON MAXSECTIONOFF MIN
=TT
*sin^h
TT T,,OFii
ON,ii,ON MAX
=
FSECTIONON
=a
(4)
(5)
(6)
Therefore,
applying the indicated equations and sending to
the Arduino output pins the high and low values, as appropriate
during the calculated times, the pulse train will be
generated. Figure 2 shows the pulse train generated for
15 pulses.
Control and Output Voltage
To control the inverter according to the established model,
two parameters must be regulated: frequency and amplitude.
The output frequency of the inverter depends on the
duration of each section, therefore, to control its value, it
will need to act on the value of TSECTION. In the same way,
the amplitude of the generated voltage will depend on the
width of the pulses.
The width of all pulses is related to the parameter
TON,MAX, which is the maximum value. Based on equation
(5), the pulses will have the maximum possible width for
each angle, hence, the output voltage generated will also
be maximum. Therefore, to control the output voltage, it
will be necessary to act on the value of TON,i, in a way that
varies depending on TON,MAX and the output voltage. For
this purpose, a new variable is introduced, VCONTROL, with
values between 0.05 and 0.95. The possible values for this
new variable will determine the regulation capacity of the
proposed model.
In summary, by acting only on two parameters (TSECTION
and VCONTROL), the output voltage and frequency can be
regulated, as well as the inverter soft start. In this type
of inverters, in which the output voltage is induced in a
transformer and filtered with capacitors, it is necessary to
perform a soft start of the inverter, in order to precharge
these components and avoid high currents. The inverter is
started up in such a way that the voltage is as low as possible
and later it is gradually increased until the set point
is reached.
The reference of the output voltage is compared
with a reference value (refAc), which has been previously
determined experimentally for the desired output
voltage. If the value acquired from A2 port is greater
than the set reference, the pulse width must be reduced
and vice versa.
Thus, with the following instructions, the output voltage
control parameter is adjusted in order that the output
voltage reaches the reference value.
V_AC = analogRead(A2);
if (V_AC < refAc && V_CONTROL > 0.05) {
V_CONTROL = V_CONTROL - 0.05;
}
else if (V_AC > refAc && V_CONTROL < 0.95) {
V_CONTROL = V_CONTROL + 0.05;
}
This parameter is applied to the calculation of the width of
each pulse at the time of calculation, where i is the corresponding
pulse:
T_ON = 1 + T_ON_MAX * (1 - V_CONTROL) * sin
(i* PI/180);
In setup, the value of VCONTROL is set to 0.95 in such a way
that in the first half of the cycle TON takes its minimum
values. Once the first half-cycle is executed, as the output
voltage is lower than the setpoint, the value of VCONTROL is
reduced by 0.05 units, increasing the TON value and
increasing the inverter output voltage. The value of VCONTROL
is reduced repeatedly until the inverter voltage
reaches the setpoint. In this mode, the inverter performs a
soft start.
The range in which the VCONTROL parameter is changed
determines how quickly the output voltage is regulated.
That is, the greater the range, the better the dynamic
response of the inverter. However, the output voltage regulation
will be less accurate. Therefore, it is appropriate to
choose an intermediate value that provides an acceptable
solution for both functions.
Frequency
In the proposed model, each half-cycle time depends on the
number of instructions to be executed, the number of
Tsection
Tsection
Tsection
0°
αi-1
αi
αi+1
FIG 1 TON and TOFF in different sections.
180°
FIG 2 15 pulses SPWM train.
December 2021 z IEEE POWER ELECTRONICS MAGAZINE 25
Ton, i-1
Toff, i-1
Ton, i
Toff, i
Ton, i+1
Toff, i+1
IEEE Power Electronics Magazine - December 2021

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